1. Field of the Invention
The present invention relates to a vehicle, generally known as a hybrid vehicle, which comprises, as power sources, an engine and a motor, the motor also being capable of generating electrical energy, the vehicle obtaining driving power from at least one of the engine and the motor.
2. Description of the Related Art
In recent years, in order to conserve fuel in the running of engines, or to reduce exhaust gas produced by combustion of fuel, hybrid vehicles have been developed, each of which comprises an engine and a motor, the motor also being capable of generating electrical energy (hereinafter referred to as a motor-generator) that are connected to a power transmission mechanism connected to driving wheels, and in which the motor-generator produces supplemental driving power depending on demand during normal travel of the vehicle, and the motor-generator is connected to the driving wheels so as to convert a portion of the kinetic energy of the vehicle into electrical energy, which is generally known as “regenerated energy”, and which is stored in a battery device (refer to, for example, Japanese Unexamined Patent Application, First Publication No. Hei 11-350995).
In such hybrid vehicles, a hybrid vehicle is also known, in which an engine and a motor-generator are directly connected to each other. Conventionally, in such a hybrid vehicle, the motor-generator is not used solely for driving the vehicle (hereinafter this driving mode is referred to as a “motor cruise mode”) because, in a motor cruise mode, the engine is a portion of the load for the motor-generator, and the motor-generator must produce power to compensate for the pumping loss and friction of the engine; therefore, fuel efficiency may not be improved when compared with the case in which the engine is used solely for driving the vehicle (hereinafter this driving mode is referred to as an “engine cruise mode”).
In the next development stage, a technology for reducing pumping loss of an engine has been developed, in which the operations of intake and exhaust valves of an engine are temporarily suspended. In the case of hybrid vehicles, it has been discovered that fuel efficiency can be improved by temporarily suspending the operations of intake and exhaust valves of an engine during a motor cruise mode so as to reduce pumping loss of the engine, even though the motor-generator is used solely for driving the vehicle and for compensating for engine friction when compared with the case of an engine cruise mode.
A hybrid vehicle, in which a motor is capable of driving the vehicle while stopping the operations of intake and exhaust valves of an engine, is disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 09-4479.
In such a hybrid vehicle which may be driven by a motor, a problem is encountered in that an unexpected deceleration feeling or jolt (hereinafter referred to as a drag feeling) is experienced when the driving mode of the vehicle is switched between the motor cruise mode and the engine cruise mode, alternately.
More specifically, when the driving mode of the vehicle is switched from the engine cruise mode to the motor cruise mode, a drag feeling is experienced due to cut of fuel supply to the engine along with a jolt (vehicle vibration) due to changes in the engine friction by stopping the operations of the intake and exhaust valves. On the other hand, when the driving mode of the vehicle is switched from the motor cruise mode to the engine cruise mode, another drag feeling is experienced due to changes in the engine friction by starting the operations of the intake and exhaust valves along with a combustion initiation jolt due to start (ignition) of the engine operation.